1. Field of the Invention
The present invention relates to a spring steel to be made into springs as automotive parts in engines, clutches, fuel injectors, suspension systems, etc. The present invention relates also to a process for producing steel wire rods for springs from said spring steel.
2. Description of the Related Art
Wires for springs in various uses as mentioned above are produced by drawing which usually follows shaving (to remove surface defects, such as flaws and decarburized layer, of rolled wire rods) and refining by lead patenting.
In pursuit of higher productivity, attempts are being made to increase the drawing speed. However, drawing at high speeds excessively increases loads on the shaving tool or chipper to damage it and results in incomplete shaving because of inadequate chip removal. There is a demand for a high-strength spring steel which does not present such drawbacks at the time of shaving.
One way to address the problem is by low temperature annealing which is intended to reduce the strength of steel. However, the problem still remains unsolved. Solutions to the problem need knowledge about the metallurgical structure and mechanical properties of the steel to be shaved. Such knowledge has never been sought, however. Meanwhile, in the case where very high fatigue strength is not required, rolled wire rods are drawn into wires directly without being shaved. (Drawing in this way is referred to as green drawing.) Wire rods for this purpose are produced by any of several ways disclosed in Japanese Patent Laid-open Nos. 116727/19982, 118013/19985, and 79719/1991.
New technologies for spring production have been proposed which supersede lead patenting that follows shaving. They include (A) heating at 450-750xc2x0 C. for a short time just enough to soften the hard surface layer, as disclosed in Japanese Patent Laid-open No. 188745/1995, and (B) heating in a gas phase at a temperature (T) of 823-973 K for a prescribed period of time (t minutes) such that Txc3x97t=6700-12000 (Kxc2x7minxc2xd), as disclosed in Japanese Patent Laid-open No. 311547/1996. Heat treatment in these manners leaves the as-rolled structure almost intact. Under these circumstances, spring steels are required to have good shaving properties as well as ability for green drawing from wire rods having the as-rolled structure.
Such technical requirements force wire producers to be ready for drawing regardless of whether shaving is carried out or not. Conventional technologies do not provide spring steels capable of both shaving and green drawing, and no attention has been paid to such products.
The present invention was completed in order to tackle the above-mentioned problems. It is an object of the present invention to provide a spring steel which has both good shaving properties and capability of green drawing, which are important in the production of springs. It is another object of the present invention to provide a process for producing wire rods for good springs from said spring steel.
The gist of the present invention resides in a spring steel having the following mechanical properties in its as-rolled state before shaving, said spring steel being optionally softened under the following conditions after rolling.
Mechanical Properties
Tensile strength (maximum)xe2x89xa61200 MPa
30% (minimum)xe2x89xa6reduction of areaxe2x89xa670% (maximum)
Low Temperature Annealing Conditions
Heating in a gas phase at a temperature (T) of 873-1023 K for a prescribed period of time (t minutes) such that Txc3x97t=7300xcx9c15000 (Kminxc2xd).
In addition, the spring steel according to the present invention should preferably meet the following conditions.
(1) It is composed of pearlite alone or ferrite and pearlite together and has a structure such that the fraction of supercooled structure is less than 10%.
(2) It has a Vickers hardness in the plane of its cross-section whose standard deviation ("sgr") is smaller than 20.
(3) It has a Vickers hardness in the plane of its cross-section which is smaller than 380.
The gist of the present invention resides also in a process for producing wire rods for springs from said spring steel, said process comprising drawing, shaving, and oil tempering, which are carried out sequentially, or comprising drawing, shaving, any of the following treatments (a) to (c), and oil tempering, which are carried out sequentially.
(a) Lead patenting treatment.
(b) Heat treatment in a gas phase at a temperature (T) of 823-973 K for a period (t minutes) such that Txc3x97t=6700xcx9c12000 (Kxc2x7minxc2xd).
(c) Heat treatment at 450-750xc2x0 C. for a short time just enough to soften the surface hard layer.